1. Field of the Invention
This invention relates to a gate-controlled bi-directional semiconductor switching device such as a triac, and more particularly to an improvement in the gate sensitivity thereof.
2. Description of the Related Art
A conventional triac which is one type of a bi-directional semiconductor switching device is formed to have such a cross section as shown in FIG. 1. In FIG. 1, 30 denotes N-type layer, 31 and 32 P-type layers, and 33, 34, 35 and 36 N-type layers. Electrode T1 is formed in contact with P-type layer 31 and N-type layer 33, gate electrode G is formed in continuous contact with N-type layer 34 and P-type layer 31, and electrode T2 is formed in continuous contact with P-type layer 32 and N-type layers 35 and 36.
Gate electrode G and a portion of P-type layer 31 lying under gate electrode G constitute the gate structure of a thyristor, and a remote gate structure is formed of an NPN transistor structure which is constituted by N-type layer 33, P-type layer 31 and N-type layer 30 and an NPN transistor structure which is constructed by N-type layer 34, P-type layer 31 and N-type layer 30. Further, N-type layer 34 and P-type layer 31 constitute a junction gate structure.
Four modes I, II, III and IV are provided to turn on the triac with the construction described above. In mode I, the gate structure of the thyristor is used to turn on the triac. That is, the triac is turned on by applying a positive trigger to gate electrode G when electrodes T1 and T2 are respectively set at positive and negative potentials. In mode II, the junction gate structure is used, and the triac is turned on by applying a negative trigger to gate electrode G when electrodes T1 and T2 are set respectively at positive and negative potentials. In mode III, the remote gate structure is used, and the triac is turned on by applying a negative trigger to gate electrode G when electrodes T1 and T2 are set respectively at negative and positive potentials. Further, in mode IV, the remote gate structure is used, and the triac is turned on by applying a positive trigger to gate electrode G when electrodes T1 and T2 are set respectively at negative and positive potentials.
In order to enhance the gate sensitivity of the conventional triac shown in FIG. 1, it is necessary to reduce an invalid current component or current which flows along the surface of a P-type base formed of P-type layer 31 and which is not contributed to an injection current. For this purpose, it has been effected to increase the resistance of a surface layer of P-type layer 31 by lowering the impurity concentration of the surface layer, or from an N-type diffusion layer which functions as a barrier interrupting the current flow in P-type layer 31. However, in either case, the gate sensitivity cannot be enhanced without deteriorating other main characteristics. For example, increase in the gate sensitivity is accompanied by deterioration in high temperature characteristics, reduction in the withstanding amount of dv/dt at the time of commutation and the like. Further, from the standpoint of the operation principle of the triac, an N-type emitter formed of N-type layer 33 must be formed in the shorted structure, thereby limiting the high sensitivity attained by the fine control for the impurity diffusion.
Under this condition, it is difficult to enhance the gate sensitivity of the conventional triac to such a degree that the triac can be directly driven by an output from a semiconductor integrated circuit (IC).
As described above, in the conventional gate-controlled bi-directional semiconductor switching device, it is difficult to enhance the gate sensitivity without lowering the characteristics such as withstanding amount of dv/dt.